1. Field of the Invention
This invention relates to semiconductor device fabrication, and in particular a method of forming two levels of metal interconnection.
2. Description of the Prior Art
The prior state of the art of forming metal interconnections in integrated circuits is reviewed in, for example, "Metalization for Very Large Scale Integrated Circuits" by P. B. Ghate, on pages 359-383 in Thin Solid Films, Volume 93, 1982. In summary, the circuit consists of a semiconductor body in which active and passive devices such as transistors, diodes, and resistors are fabricated. These devices are interconnected by narrow lines of a thin metal layer. For cost savings and for improved performance, the size of the circuit chip is reduced by using two levels of metal interconnections. The first layer is usually aluminum, which has very high conductivity and can be deposited very easily. The interlevel insulator is usually silicon dioxide, silicon nitride, or polyimide. The contact apertures in the insulator are opened by photolithography and wet etching or plasma etching. The second level metal is mostly aluminum, but titanium-aluminum, titanium-gold, etc., double layers have been used. The second level metal is also patterned by standard techniques. Although there are several aspects affecting the reliability of the contacts between the two levels of the metal, one major problem has been the presence of an unwanted layer of an insulating residue on the surface of the first level metal at the bottom of the aperture. This residue layer is believed to contain mostly aluminum oxide, aluminum hydroxide, or hydrocarbons and fluorocarbons left from a plasma etch process. The removal of this layer by wet etching or sputter etching has been the subject of several investigations and inventions. (See, for example, Nasa Tech Brief #MFS-25149; J. Electronic Materials, Vol. 11, No. 3, p.441, 1981; J. Electrochemical Society, Vol. 125, No.3, p.467, 1978; J. Vacuum Science and Technology, Vol. 20, No.3, p.396, 1982; U.S. Pat. No. 4,184,933; U.S. Pat. No. 4,164,461). The use of wet etching for the cleaning of the contact apertures has been found to be disadvantageous because it attacks lower aluminum and wet rinses finally leave an oxide layer on aluminum. Sputter etching, which is extensively used in experimentation, results in radiation damage and is unsuitable for certain types of circuits. Prior to the present invention, there has not been a high yield reliable process for making good ohmic contact between the lower level aluminum layer and an upper level metal layer through a contact aperture.